The Huygens Atmospheric Structure Instrument (HASI) is a
multi sensor package which has been designed to measure the
physical quantities characterizing Titan's atmosphere during
the Huygens entry and descent phases and at the surface.
HASI's suite of sensors measured the physical and electrical
properties of Titan's atmosphere. Profiles of temperature,
pressure, density, atmospheric conductivity have been
collected. HASI investigated the electric properties and the
nature of the surface. Accelerometers measured deceleration
in all three axes as the probe was in the entry phase. With
the aerodynamic properties of the probe already known, it
has been possible to determine the density of Titan's high
atmosphere and to deduce temperature and pressure profiles.
During the descent (from 162 km down to the surface),
temperature and pressure of the atmosphere were measured
directly and the Permittivity and Wave Analyzer sensors
measured the electron and ion conductivities as well as the
complex permittivity of the atmosphere and searched for
electric wave activity including lightning. Acoustic signals
collected by the HASI microphone were processed by the on
board FFT and their amplitude and main frequency, averaged
over two minutes, were transmitted to the ground. The HASI
instrument package had a total mass of 5.7 kg. The
accelerometers package is mounted at the center of mass of
the probe and the temperature sensors and the pressure probe
inlet are mounted on a stem outside of the probe. Two
deployable booms carrying Permittivity ad Wave electrodes
stowed under the thermal shield of Huygens, has been
released at the beginning of the descent into Titan's
atmosphere. HASI provided, in addition to its own scientific
data, calibration information of use by other instruments on
Huygens and by remote sensing observations from the Cassini
orbiter.

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